In automatic testing systems many applications require programmable sources of voltage or current as supplied by a programmable power supply. Such power supplies are generally responsive to analog control voltage which may be within a given range and of a predetermined polarity. For example, a power supply being programmed for a controlled output voltage may require an input voltage positive with respect to common and in the range of 0 to 10 volts while another power supply may be programmed for a controlled output current negative with respect to ground requiring a negative voltage with respect to common in the range from 0 to 1 volts. In either case digital voltage or current control signals have been converted to an analog voltage in the range of 0 to 10 volts and positive with respect to common. Thus, digitally controllable interface means is required to determine the range of voltage and polarity to be applied to the power supplies in response to the 0 to 10 positive voltage.